Conventional processes for the preparation of a water soluble cationic polymer useful as a flocculent include polymerization in an aqueous solution, water-in-oil emulsion polymerization, and suspension polymerization in a hydrophobic solvent. Others have prepared a water soluble, anionic polymer by precipitation polymerization in an aqueous solution of ammonium sulfate. Still others have carried out the polymerization in an aqueous solution in the presence of a polyhydric alcohol or a polyelectrolyte as a dispersant.
U.S. Pat. No. 4,929,655 (Takeda et al.), which issued on May 29, 1990, and which is incorporated herein by reference, provided a novel process for preparing a water soluble polymer dispersion which overcomes the many disadvantages of the stationary polymerization, water-in-oil type emulsion polymerization and suspension polymerization processes. This process for the production of a water soluble dispersion includes the polymerization of water soluble monomers of 5 to 100 mole % of a cationic monomer represented by the following formula (I), 0 to 50 mole % of another cationic monomer represented by the following formula (II) and 0 to 95 mole % (meth)acrylamide in the presence of 1 to 10% by weight of an organic high molecular multivalent cation comprising a water soluble polymer containing at least a monomer of formula (II) (i.e., a hydrophilic monomer), based on the total weight of the monomers, in an aqueous multivalent anionic salt solution having a concentration of 15% by weight or more. The first cationic monomer discussed above can be represented by the following general formula (I): ##STR2## where R.sub.1 is either hydrogen or CH.sub.3 ; R.sub.2 and R.sub.3 ar each an alkyl group having 1 to 3 carbon atoms; A.sub.1 is either an oxygen atom or NH; B.sub.1 is either an alkylene group having 2 to 4 carbon atoms or a hydroxypropylene group, and X.sub.1.sup.- is an anionic counterion, and/or a second cationic monomer represented by the following general formula (II): ##STR3## where R.sub.4 is either hydrogen or CH.sub.3 ; R.sub.5 and R.sub.6 are each an alkyl group having 1 to 2 carbon atoms; R.sub.7 is a hydrogen atom or an alkyl group having 1 to 2 carbon atoms; A.sub.2 is either an oxygen atom or NH; B.sub.2 is either an alkylene group having 2 to 4 carbon atoms or a hydroxypropylene group and X.sub.2- is an anionic counterion.
U.S. Pat. No. 5,006,590 (Takeda et al.), which issued on Apr. 9, 1991, and which is incorporated herein by reference, is similar to Takeda '655, except that polymerization is carried out in the presence of both: (1) a water soluble cationic polymer which is insoluble in an aqueous solution of a polyvalent anionic salt (seed polymer); and (2) a water soluble cationic polymer which is soluble in an aqueous solution of a polyvalent anionic salt (dispersant polymer). The water soluble cationic polymer that is insoluble in the aqueous solution of polyvalent anionic salt contains at least 5 mole % of cationic monomer units represented by the aforementioned general formula (I) above and the water soluble cationic polymer that is soluble in the aqueous solution of a polyvalent anionic salt contains at least 20 mole % of cationic monomer units represent by the general formula (II) above.
Although the final polymer dispersion viscosities were satisfactory, i.e., 1,000 cp or below, the processes disclosed in U.S. Pat. Nos. 4,929,655 and 5,006,590 proceed through very high process viscosities (i.e., &gt;100,000 cp and usually as high as 2,000,000 cp), which require the use of a custom built high viscosity polymerization reactor. Due to these high process viscosities, the methods disclosed in U.S. Pat. Nos. 4,929,655 and 5,006,590 can only be used in the synthesis of polymer systems containing low polymer contents (i.e., 15 to 20%).
The present inventors have discovered through extensive experimentation, that hydrophobically modified diallyldimethylammonium chloride (DADMAC) polymers can be used in combination with the dispersant polymers disclosed in U.S. Pat. Nos. 4,929,655 and 5,006,590 as colloidal stabilizers in the dispersion polymerization process. The dispersant system of the invention is highly soluble in water and at least partially soluble in aqueous salt solutions. The dispersant system has been demonstrated to facilitate the precipitation process and thereby very significantly reducing the process viscosity. This allows the production of high molecular weight polymer dispersions in a conventional latex-type reactor, thereby avoiding the substantial costs associated with high viscosity custom built polymerization reactors. Further, the reduction in process viscosity will allow the preparation of polymer systems containing higher actives (such as, from 15 to about 30-40 percent by weight) in latex-type reactors.
Moreover, it has also been discovered that even lower production viscosities are maintained through subsequent polymerizations when from about 1.5 to about 12 percent by weight of polymer from one polymerization process is used as the seed in the next polymerization generation of the same polymer. It should be noted that it has been determined that when the dispersant polymers of the invention used independently of each other, production viscosity increases with each successive generation, until, process viscosity becomes so great that standard latex-type reaction will not accommodate the process. The present inventors have also determined that the multivalent anionic salt concentration can be optimized to control process viscosity.
The present invention provides many additional advantages which shall become apparent as described below.